Project Summary Myelination is critical for normal conduction of action potentials and synchronized transmission of neural impulses. Recent studies have demonstrated that CNS myelin development, maturation and maintenance are regulated by neuronal activity and experience. In contrast, very little is known about how experience and activity regulate myelin development in the PNS, nor how experience-dependent neural activity affects re- myelination after peripheral nerve or glial cell injury in vivo. This is in part due to the difficulty in precisely altering activity of PNS nerves, since they are typically a mixture of both sensory and motor fibers, or of sensory axons mediating different types of modalities. We hypothesize that sensory activity regulates peripheral nerve myelination, myelin maintenance and remyelination, and propose to address this fundamental gap in knowledge using the auditory system as a model. Type I auditory nerve (AN) fibers in the mouse cochlea offer an ideal platform to dissect the role of activity on peripheral myelination because: (a) maturation of AN myelin coincides with auditory function maturation during the first postnatal month, suggesting that activity may affect Schwann cells; (b) re-myelination of AN fibers occurs following Schwann cell ablation; (c) AN myelin dysfunction is associated with hearing deficits, demonstrating a critical role for myelination in cochlear function, particularly in the transmission of key temporal features of sound that are important for understanding speech; (d) we can manipulate the activity of primary auditory neurons by ablating hair cells, by exposing animals to defined auditory experiences, or using genetic tools; (e) we can efficiently isolate and examine the entire peripheral AN at the structural, cellular and molecular levels; and (f) we can assess the impact of myelin defects on cochlear and auditory nerve function in the intact mouse can be assessed at high- resolution with standard electrophysiological techniques. Furthermore, AN axons are myelinated by both Schwann cells (in the distal part) and oligodendrocytes (in the proximal part), permitting a direct comparison of the effects of activity on both types of myelinating cells within the same nerve. We will use mouse models to address this gap in knowledge in three specific aims. In Aim 1, we will use mutants that are defective in hair cell mechanotransduction and synaptic function to determine the role of AN activity in myelination during the neonatal and juvenile time periods. In Aim 2, we will test the roles of auditory experience and NRG1/ErbBR signaling in AN myelination during the neonatal and juvenile periods. Finally, in Aim 3, we will test whether sound-driven neuronal activity modulates AN re-myelination in the mature cochlea. Successful completion of the proposed aims will provide valuable insight into the role of neural activity in PNS myelination and a more precise understanding of the impact of myelin dysfunction on hearing.